As is well known, a synchronous transmission system (STS) is widely utilized to accommodate as many voice/data channels as possible in a given communication path. According to a synchronous digital hierarchy (SDH) recommended by the International Telecommunications Union-Telecommunications Sector (ITU-TS), digital signal level 1 (DS-1) data of the North America or digital signal level 1 data of Europe (DS-1E) is converted to synchronous transport module level 1 (STM-1) data through the use of various data packets of containers, virtual containers, tributary units, tributary unit groups (TUG's) and administrative units, wherein the DS-1and DS-1E data represent 1.544 Mbps pulse code modulated (PCM) serial data having 24 voice channels and 2.048 Mbps PCM serial data containing 32 voice channels, respectively.
In order to efficiently perform the conversion operation, there are generally provided two TUG clock signals in the STS, wherein the two TUG clock signals are of a same predetermined identical clock frequency, e.g., 6 MHz, and one of them is used as a system main clock signal. In a normal condition, the system main clock signal is used for driving the STS and the other TUG clock signal is a stand-by clock. If the system main clock signal is detected to be in failure, the STS immediately replaces the failed TUG clock signal with the other TUG clock signal to continue the conversion process by using the replaced TUG clock signal. In changing the failed TUG clock signal with the other TUG clock signal, a clock failure detector which monitors two TUG clock signals and produces a clock failure signal for each of the TUG clock signals, is used for denoting a changing time.
In one of the conventional clock failure detection schemes, the failure of the TUG clock signal is detected by using a reference clock signal generated from a local oscillator included in the system, that is, the TUG clock signal is considered to be in failure if it is not supplied to the clock failure detector for a predetermined time interval, wherein the time interval is measured by using the reference clock signal generated from the local oscillator and its length depends on performance requirements of the STS.
In another conventional clock failure detection scheme, the failure of a TUG clock signal is detected by using the other TUG clock signal as the reference clock signal, i.e., the predetermined time interval is measured by using the other TUG clock signal.
Since, however, in the first method, providing the local oscillator to the system makes the overall circuit very complex and also exacts a high production cost, because the local oscillator has a crystal resonator, a capacitor, etc., and in the second method, the failures of two TUG clock signals cannot be detected when both of the two TUG clock signals become erroneous at the same time since there will be no proper reference clock working.